Contactless potentiometer with automatically controlled light source

ABSTRACT

The arrangement includes a light source and a photosensitive unit positioned to receive light from the light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from the light source. A light-blocking arrangement, for example in the form of a film strip, is positioned intermediate the light source and the photosensitive unit, and is mounted for displacement relative to the photosensitive unit and light source in a predetermined direction along a predetermined path. The light transmittance of the light-blocking arrangement varies in said predetermined direction, so that as the light-blocking arrangement is displaced in said direction the luminous flux received by the photosensitive unit from the light source will vary accordingly. A compensating arrangement establishes a linear relationship between the displacement of the light-blocking arrangement and the magnitude of the light-dependent signal, by automatically varying the luminous flux emitted from the light source towards the photosensitive unit in dependence upon the position of the light-blocking arrangement.

United States Patent Bachmann July 1, 1975 l l CONTACTLESS POTENTIOMETER WITH Primary Examiner-Walter Stolwein AUTOMATICALLY CONTROLLED LIGHT Attorney, Agent, or FirmMichael S. Striker SOURCE 57 ABSTRACT [75] Inventor: Hans Bachmann, Zug, Switzerland 1 The arrangement includes a hght source and a photol Asslgneei lmel'madm AG" g Switzerland sensitive unit positioned to receive light from the light [22] Filed: Mar. 19, 1974 source and operative forgenerating an electrical output signal having a magnitude dependent upon the lul l pp P10545255) minous flux received from the light source. A lightblocking arrangement, for example in the form of a [30] Foreign Applicafion Priority Data film strip, is positioned intermediate the light source M 2' 973 German 2313997 and the photosensitive unit and 18 mounted for dlS- y placement relative to the photosensitive unit and light 0 source in a predetermined direction along a predeterfi 250/211 mined path. The light transmittance of the light- 5 K 237 R blocking arrangement varies in said predetermined di- 1 0 234 R rection, so that as the light-blocking arrangement is displaced in said direction the luminous flux received by the photosensitive unit from the light source will [56] Reerences cued vary accordingly. A compensating arrangement estab- UNITED STATES PATENTS lishes a linear relationship between the displacement 3.087.069 /1 3 MonCrieff-Yeates .7 250/2ll K of the light-blocking arrangement and the magnitude 3,449,585 6/l969 Trehub i. 250/205 f h light-dependent signal, by automatically varying 3,539.8l6 ll/l970 Chamberlin 250/211 K the luminous flux emitted from the light source -E: 32 3 wards the photosensitive unit in dependence upon the 5715617 ll/l973 Dubatis l t ti s ili.........l....::::::i:: 250/205 posifio of the ammgemw" 19 Claims, 5 Drawing Figures 1 CONTACTLESS POTENTIOMETER WITH AUTOMATICALLY CONTROLLED LIGHT SOURCE BACKGROUND OF THE INVENTION The invention relates to a contactless potentiometer comprised of a light source and a light-responsive receiver, preferably a photoelement, with a lightblocking arrangement of varying light transmittance being arranged in the path of light from the source to the receiver and displaceable so as to vary the luminous flux received by the receiver.

Arrangements constituting the functional equivalents of potentiometers, but not making use of wiper contacts, are already known, for example from US. Pat. No. 3,539,816 and from the article Using photocells for electro-optical potentiometer" by Brown and Tomasulo appearing in the publication Electronic Products," Sept. 1969 issue, pp. 150-151.

in the arrangement disclosed in the aforementioned U.S. patent, an inner hollow cylinder is received in an outer hollow cylinder. Provided inside the inner cylinder is a lamp, and the inner cylinder is slitted. The inner surface of the outer cylinder is of light-sensitive semiconductor material. An external voltage is applied to the arrangement, and if then the inner cylinder is turned a variable voltage output is obtained, with the important concomitant advantages of little or no wear of moving parts, as a result of elimination of the sliding wiper, and also higher resolution, and higher speed of adjustment.

In the aforementioned article in Electronic Products," an arrangement is provided for the generation of a sawtooth waveform having steep flanks. Use is made of two photoresistors connected in series and alternately blocked from a source of light by a rotation member provided with a slit-shaped aperture, so as to generate the desired sawtooth waveform.

However, the known contactless potentiometers are not completely satisfactory, because the relationship between the magnitude of the output voltage and the displacement (rotational or translatory) of the lightblocking member is not sufficiently linear. This results from non-linearities in the response of the photoelements employed and from the temperature dependence of the response of such elements. Another disadvantage, particularly when use is made of photoresistors, is the so-called transient effect," i.e., if the contactless pentiometer is left in one position for a long period of time, and if the potentiometer setting is then changed, a certain time passes before the output voltage leaves the previous value.

A further disadvantage of known constructions is that fluctuations in the luminous flux emitted by the light source, aging of the light source, and other such effects, have a direct and detrimental effect upon the accuracy of the instrument.

SUMMARY OF THE INVENTION It is a general object of the invention to provide a socalled contactless potentiometer" not characterized by the disadvantages described above.

It is a more particular object to provide an arrangement which exhibits all the advantages of the known Constructions, while also exhibiting an exactly linear relationship between the rotational or translatory displacement of the moving portion of the instrument and the magnitude of the output voltage, as well as temperature independence and independence from the effects of light source aging and fluctuations of the voltage supply of the arrangement.

This object and others which will become more understandable from the following description of a specific embodiment, can be met according to one advantageous concept of the invention, by providing an arrangement comprising, in combination, a light source and photosensitive means positioned to receive light from the light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from the light source. The arrangement further includes light-blocking means positioned intermediate the light source and the photosensitive means and mounted for displacement relative to the photosensitive means and the light source in a predetermined direction along a predetermined path. The light transmittance of the light-blocking means varies in said predetermined direction, so that as the lightblocking means is displaced in said direction the luminous fiux received by the photosensitive means from the light source will vary accordingly. The arrangement further includes compensating means operative for establishing a linear relationship between the displacement of the light-blocking means and the magnitude of the electrical output signal, by automatically varying the luminous flux emitted from the light source towards the photosensitive means in dependence upon the position of the light-blocking means.

According to a further concept of the invention, the compensating means comprises second photosensitive means positioned to receive light from the light source and operative for generating a second electrical signal having a magnitude dependent upon the luminous flux received from the light source, and also stabilizing circuit means connected to the second photosensitive means and to the light source and operative for maintaining said second signal constant by automatically varying the luminous flux emitted by the light source in dependence upon the magnitude of the second signal.

According to a still further concept of the invention, the compensating means further includes second lightblocking means positioned intermediate the light source and the second photosensitive means and mounted for displacement relative to the second photosensitive means and the source in a predetermined direction along a predetermined path, the second lightblocking means being coupled to the first lightblocking means for displacement jointly therewith, and the light transmittance of the second light-blocking means varying in the direction in which the second light-blocking means is displaceable.

For example, the variation in the light transmittance of the first light-blocking means, in the direction of displaceability of such means, may be linear, with the output signal produced by the respective first photosensitive means being (in the absence of the remainder of the arrangement) only approximately linearly related to the displacement of the first light-blocking means, on account of non-linearities in the response of the first photosensitive means. The variation in the light transmittance of the second light-blocking means, in the direction of displaceability of such means, can then be made such that the effect thereof upon the luminous flux emitted by the light source will compensate for such non-linearities and establish an exactly linear relationship between the displacement of the first lightblocking means and the magnitude of the first electrical signal.

An advantage of such an approach is that, in addition to compensating for the nonJinear response of the photosensitive means, the luminous flux emitted from the light source is stabilized, despite for example aging of the light source or fluctuations of the supply voltage of the light source; in particular use can be made of a Zener diode or the equivalent for this latter purpose.

As one possibility, the light-blocking means can be essentially comprised of film strips the light transmittance of which varies in longitudinal direction. As an alternative, however, use could be made of members having light-transmitting portions and other portions which transmit substantially no light. The term light blocking means is intended to describe both such possibilities, and others mentioned below.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 depicts the mechanical portion of one embodiment of the invention;

FIG. 2 depicts the coiled light-blocking members of FIG. 1, spread out flat;

FIG. 3 depicts a preferred location for the light source relative to the first and second photosensitive means;

FIG. 4a depicts a circuit used with the first photosensitive means; and

FIG. 4b depicts a circuit used with the second photosensitive means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before entering upon an explanation of the illustrated embodiment, it is to be noted that the basic operation of the contactless potentiometer of the exemplary disclosed embodiment resides in the fact that it is comprised of an output photocell, identified in the drawing with reference numeral 8, and illuminated by a continuously variable amount of light to cause the photocell to produce an output light-dependent current proportional to the changes of the incident luminous flux. After this light-dependent current is amplified and converted, preferably by means of an operational amplifier comprised of a negative-feedback branch, there is produced an output voltage which is proportional to the luminous flux incident upon and exciting the photocell. Below will be described the manner in which, according to the invention, the linearity of this relationship can be made extremely exact, as well as how to stabilize the entire apparatus against temperature fluctuations and aging effects.

As shown in FIG. I, the potentiometer arrangement has a rotary support shaft 3 mounted for rotation at both ends by means of supports 4, 5. Fixedly connected in which are received the edges of the light-blocking members 9, 10. In this embodiment, film strips 9, 10 are employed for the light-blocking members; however, other light-blocking members and means of different configuration could evidently be employed. The film strips 9, 10 are coiled into a generally cyclindrical shape and supported on the facing inner sides of the disks 1, 2 to form a half shell mounted on the potentiometer shaft 3.

FIG. 2 depicts the film strips 9, 10 developed, or laid out flat. The film strip 9 is associated with the measuring photoelement 8, whereas the film strip 10 is associated with the reference photoelement 7. Both the film strips 9, l0 and the photocells 8, 7 are so mounted as to be somewhat spaced from each other in axial direction. To shield against stray light, use in advantageously made of a separator element 21 arranged between the spaced elements. As shown in FIG. 1, the photoelements 8, 7 are arranged above the mounting arrangement for the film strips 9, l0 and above the film strips themselves. A light source 6 is positioned in the interior of the cylindrical half shell comprised of the film strips 9, l0 and the facing surfaces of the disks 1, 2.

In order to maintain the two photocells 8, 7 at substantially the same temperature, they are both mounted on a metal block 11 of high thermal conductivity; however, it is also possible with modern technology to form the photoelements from a single piece of semiconductor material, preferably silicon.

As FIG. 2 shows, the film strip 9 associated with the output photocell 8 is provided with two identical black wedges 22. It is clear that when the potentiometer shaft 3 is turned by a predetermined angle, the luminous flux incident upon the output photocell 8 and thereby also the output current of the photocell will change in proportion, which is the desired relationship for such an instrument.

FIG. 4a depicts the output photocell 8 connected to the input of an operational amplifier 12 provided with a negative-feedback resistor 13. The amplifier 12 amplifies the light-dependent current of photocell 8, to produce a light-dependent output voltage UA.

Cooperating with the portion of the arrangement described thus far is the reference photoelement 7 and the associated film strip 10, as well as a stabilizing circuit shown in FIG. 4b, the purpose and operation of which will be described further below.

FIG. 2 depicts the reference film strip 10, composed of the same material as the film strip 9. The strip 10 is not opaque to the same extent as the strip 9, but instead is opaque only at the right-hand edge portion thereof, this edge portion being provided with an opaque portion 23 of variable breadth.

It is to be understood, with respect both to the film strip 9 and the film strip 10, that the darkened portions 22 and 23 thereof can be entirely opaque, i.e., transmit zero light, or can be merely of a constant light transmittance markedly less than that of the remaining portions of the film strip. As a further possibility, and particularly in the case of film strip 9, the strip need not be sharply divided into the depicted wedge-shaped portions, but instead could have a continuously varying light transmittance which is constant over the entire width of longitudinally successive portions of the strip. Likewise, if the strip be divided into discrete areas of two different light transmittances, as with the strip 9, the areas need not be as shown, but only need be such that effective light transmittance relative to the respective photocell varies linearly or approximately linearly.

The output photoelement 8 and the reference photoelement 7 are illuminated from the same light source 6. The output current of reference photoelement 7 will vary in response to changes in the luminous flux incident thereupon, and it will be evident that rotation of the potentiometer shaft 3 will tend to produce such changes, although such changes will be very shortlasting for reasons explained below.

The reference photoelement 7 is connected in the circuit of FIG. 4b. The circuit of FIG. 4b is operative for varying the luminous flux emitted by the light source 6 in such a manner as to maintain constant the light-dependent output current of the photoelement 7. To this end, the circuit of FIG. 4b is comprised of an operational amplifier 14, 15, the output of amplifier 14 being very loosely coupled to the input thereof by a negative-feedback resistor 15. Applied to the operational amplifier input is the output current I,, of the reference photoelement 7. ln known manner, it is assumed in the following that the operational amplifier is comprised of an infinite-gain amplifier, the output of which is coupled back to the input by means of a negative-feedback circuit branch, the input current of the operational amplifier being negligible compared to the output current thereof. Also applied to the operational amplifier input is the current i furnished by a reference circuit stage. The two currents I and i are of opposite polarity, so that the difference between their magnitudes constitutes a resultant input current for the operational amplifier. The reference circuit stage includes a Zener diode l8, provided to afford a voltage of constant magnitude irrespective of changes in the magnitude of the supplied voltage, the cathode of the Zener diode 18 being grounded and the anode thereof being connected by means of a resistor 17 to a source of negative voltage U,. The reference current i flows through a resistor 16 connected between the Zener diode anode and the operational amplifier input.

Since as assumed, the input current of the operational amplifier 14 is negligibly small, and since the negative feedback coupling afforded by resistor 15 is relatively large, relatively small changes in the difference current 1., 1,, produce a marked change of output voltage. so that even a very small change of the light-dependent current I,, of the reference photoelement 7 results in a marked compensating change of the total luminous flux emitted by the light source 6, the light source 6 being connected across the output of the operational amplifier l4, l and being controlled directly thereby.

Since the reference photoelement 7 (and also the output photoelement 8) is illuminated by the light source 6, changes in the photocurrent l,, are quickly compensated for by corresponding opposite changes in the luminous flux emitted from light source 6. With this in mind, the reference film strip 10 is provided along its right-hand edge portion with an opaque shaded portion 23 of varying width, the width of the shaded portion 23 varying in such a manner as to compensate for nonlinearities in the response of the output photocell 8. For example, if it is known that when the shaft 3 is turned to a predetermined angular position the output current increase of photocell 8 tends to be less than it should be, then the corresponding portion of reference film strip 10 can be provided with a shaded portion of the proper width. When this shaded portion blocks some of the light from source 6 from reaching photocell 7, the automatic stabilizing action of the circuit of FIG. 4b will cause a corresponding increase in the luminous flux emitted from source 6, so that the output current of photocell 7 will be maintained constant. This boost in the flux emitted from source 6 will accordingly compensate for the just-mentioned tendency of the output current of photocell 8 to increase insufficiently.

Ordinarily, the exact course of the non-linearity of the photocell 8 will not be known in advance, so that it will be of most advantage to form the shaded portion 23 on the reference film strip 10 afterwards and in view of the evaluated response of the photocell 8. After construction of the potentiometer, the shaded portion 23 on film strip 10 can be plotted, either by hand, or else by automatic means, and in such a manner as to result in the highest possible exactness for the potentiometer, which advantageously can be achieved by the use of a measuring circuit. The latter expedient has the advantage that it permits an automation of the correcting or compensating plotting operation in such a manner that the output current of the measuring circuit can be compared, in servo system fashion, against an absolutely linearly increasing signal, with the resulting error signal activating a mechanical plotting arrangement operative for laying out or plotting the shaded area 23 of the reference or compensating film strip 10. ln this manner, it becomes possible to take into account and compensate for all variables which have an effect upon the linearity of the potentiometer, such as for example nonlinearities in the wedge-shaped areas of the film strip 9, itself, non-linearities in the response of photocell 8, geometrical non-uniformities in the emission of light from the light source and finally the mechanical irregularities of the construction itself. All these factors taken together can be compensated for with the approach according to the invention.

The stabilizing of the output photocurrent of photoelement 7 will of course also occur in response to tern perature fluctuations such as might tend to affect the response of the photocell, and also in response to aging and voltage supply fluctuations, since all these factors would tend to change the value of the photocurrent l,,, and would accordingly cause the stabilizing circuit to counteract such tendencies. The person skilled in the art will understand that if the resistor 15 and accordingly the stabilizing amplification is selected large enough, even large changes in the sensitivity of the photoelement or in the lamp efficiency can be quickly compensated without marked changes in the photocur rent I,,.

Advantageously, according to the invention, for the light source 6 use is made of a light-emitting diode, preferably a gallium arsenide diode. This diode, as shown in FIG. 3, is not positioned centrally relative to the photoelement 8, 7, but instead is displaced, as also shown in FIG. 1, so that the greatest part of the emitted light 19 is incident upon the output photocell 8. This has two advantages. Firstly, as a result of this deliberately unequal light distribution, it is possible, at the outer edge of the reference film strip 10, as shown in FIG. 2, to perform linearity corrections which, in spite of the ordinarily very small extent of such corrections, can involve the use of large shaded areas, so that the plotting of the shaded area 23 can be performed much more precisely than otherwise.

Furthermore, in this manner it is possible to distribute the light incident upon the photoelements 8, 7 in correspondence to the ratio of the maximum output voltage UA of the output operational amplifier 12 to the reference output voltage on the light source 6, so as to achieve equal values for the resistors 13 and 16, which effect temperature compensation. Advantageously, the photoelements 7 and 8 are silicon photoelements, since silicon photoelements do not exhibit remanence characteristics like those of photoresistors; furthermore, they do not age to any great extent; they have a reproducible temperature dependence and are very quickly responsive.

Summarizing, the stabilization of the lightdependent output current of reference photoelement 7 makes the potentiometer output voltage UA substantially independent of temperature, independent of aging of the light source, independent of non-linearities of the photoelements, and the like, and always exactly proportional to the position of the black wedges 22, so as to be dependent only upon the rotational position of the potentiometer shaft 3.

It will be understood that instead of film strips, members of glass can be employed, or light blocking members having apertures of increasing size, or wedgeshaped blocking members, etc. Therefore, when reference is made to variations in the light transmittance of the blocking member along the direction of displaceability thereof, it is to be understood that the expression covers not only materials the light transmittance of which changes, but also covers, for example, a simple wedge-shaped member which would as it is displaced permit more and more light to impinge upon the associated photocell. Also, the displacement between the light-blocking member and the photocell and light source need not be rotational as illustrated, but could instead be translatory.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of circuits and constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a contactless potentiometer, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. An arrangement of the character described, comprising, in combination, a light source; photosensitive means positioned to receive light from said light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from said light source; light-blocking means positioned intermediate said light source and said photosensitive means and mounted for displacement relative to said photosensitive means and said source in a predetermined direction along a predetermined path, the light transmittance of said light-blocking means varying in said predetermined direction, whereby as said light-blocking means is displaced in said direction the luminous flux received by said photosensitive means from said light source will vary accordingly; and compensating means operative for establishing a predetermined mathematical relationship between the displacement of said light-blocking means and the magnitude of said signal by automatically varying the luminous flux emitted from said light source towards said photosensitive means in dependence upon the position of said light-blocking means, wherein said photosensitive means and said signal respectively constitute first photosensitive means and a first signal, and wherein said compensating means comprises second photosensitive means positioned to receive light from said light source and operative for generating a second electrical signal having a magnitude dependent upon the luminous flux received from said light source, and stabilizing circuit means connected to said second photosensitive means and to said light source, and operative for maintaining constant the magnitude of said second signal by automatically varying the luminous flux emitted by said light source, said light-blocking means constituting first light-blocking means, and wherein said compensating means further includes second light-blocking means positioned intermediate said light source and said second photosensitive means and mounted for displacement relative to said second photosensitive means and said source in a predetermined direction along a predetermined path, said second light-blocking means being coupled to said first light-blocking means for displacement jointly therewith, and the light transmittance of said second light-blocking means varying in the direction in which said second light-blocking means is displaceable.

2. An arrangement of the character described, comprising, in combination, a light source; photosensitive means positioned to receive light from said light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from said light source; light-blocking means positioned intermediate said light source and said photosensitive means and mounted for displacement relative to said photosensitive means and said source in a predetermined direction along a predetermined path, the light transmittance of said light-blocking means varying in said predetermined direction, whereby as said light-blocking means is displaced in said direction the luminous flux received by said photosensitive means from said light source will vary accordingly; and compensating means operative for establishing a linear relationship between the displacement of said lightblocking means and the magnitude of said signal by automatically changing both the luminous flux emitted from said light source and the luminous flux incident upon said light-blocking means in dependence upon the position of said light-blocking means.

3. An arrangement of the character described, comprising, in combination, a light source; photosensitive means positioned to receive light from said light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from said light source; light-blocking means positioned intermediate said light source and said photosensitive means and mounted for displacement relative to said jnotosensitive means and said source in a predetermined direction along a predetermined path, the light transmittance of said light-blocking means varying n said predetermined direction, whereby as said light-blocking means is displaced in said direction the luminous flux received by said photosensitive means from said light source will vary accordingly; and compensating means operative for establishing a predetermined mathematical relationship between the dis placement of said light blocking means and the magnitude of said signal by automatically changing both the luminous flux emitted from said light source and the lu minous flux incident upon said light-blocking means in dependence upon the position of said light-blocking means.

4. An arrangement as defined in claim 3, wherein said photosensitive means and said signal respectively constitute first photosensitive means and a first signal, and wherein said compensating means comprises second photosensitive means positioned to receive light from said light source and operative for generating a second electrical signal having a magnitude dependent upon the luminous flux received from said light source, and stabilizing circuit means connected to said second photosensitive means and to said light source, and operative for maintaining constant the magnitude of said second signal by automatically varying the luminous flux emitted by said light source.

S. An arrangement of the character described. comprising, in combination. a light source; photosensitive means positioned to receive light from said light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from said light source; light-blocking means positioned intermediate said light source and said photosensitive means and mounted for displacement relative to said photosensitive means and said source in a predetermined direction along a predetermined path, the light transmittance of said light-blocking means varying in said predetermined direction, whereby as said light-blocking means is displaced in said direction the luminous flux received by said photosensitive means from said light source will ary accordingly; and compensating means operative for establishing a linear relationship between the displacement of said lightblocking means and the magnitude of said signal by automatically varying the luminous flux emitted from said light source towards said photosensitive means in dependence upon the position of said light-blocking means, wherein said photosensitive means and said signal respectively constitute first photosensitive means and a first signal, and wherein said compensating means comprises second photosensitive means positioned to receive light from said light source and operative for generating a second electrical signal having a magnitude dependent upon the luminous flux received from said light source. and stabilizing circuit means connected to said second photosensitive means and to said light source, and operative for maintaining constant the magnitude of said second signal by automatically varying the luminous flux emitted by said light source, said light-blocking means constituting first light-blocking means. and wherein said compensating means further includes second light-blocking means positioned intermediate said light source and said second photosensitive means and mounted for displacement relative to said second photosensitive means and said source in a predetermined direction along a predetermined path, said second light-blocking means being coupled to said first lightblocking means for displacement jointly therewith, and the light transmittance of said second light-blocking means varying in the direction in which said second light-blocking means is displaceable.

6. An arrangement as defined in claim 5, wherein the relationship between the magnitude of said first and second signals and the luminous flux received by said first and second photosensitive means, respectively, is temperature-dependent, and further including means for maintaining said first and second photosensitive means at substantially the same temperature.

7. An arrangement as defined in claim 5, wherein the variation of the light transmittance of said first photosensitive means in the direction of displaceability thereof is linear.

8. An arrangement as defined in claim 5, wherein said first and second blocking means are comprised of a film strip.

9. An arrangement as defined in claim 5, wherein said first blocking means is comprised of a first film strip and wherein said second blocking means is comprised of a second film strip, and wherein said film strips are mounted on a common support structure for movement in the same direction.

10. An arrangement as defined in claim 5, wherein said first and second blocking means are respectively comprised of first and second coiled film strip portions supported at the edges thereof on two axially spaced disks mounted on a rotating shaft, said coiled film strip portions together with said disks defining a chamber, and wherein said light source is located interiorly of said chamber and wherein said first and second photosensitive means are located exteriorly of said chamber.

11. An arrangement as defined in claim 10, wherein said light source comprises a gallium arsenide lightemitting diode.

12. An arrangement as defined in claim 9, wherein said first film strip portion has a plurality of identical wedge-shaped light-blocking surface portions, and wherein said second film strip portion has along one edge thereof an elongated light-blocking surface portion of varying width.

13. An arrangement as defined in claim 5, wherein said first and second photosensitive means comprise photoelements mounted on a common metal support, and including means intermediate said first and second photosensitive means for shielding the latter from stray light.

14. An arrangement as defined in claim 5, wherein said first and second photosensitive means each comprise silicon semiconductor photoelements.

15. An arrangement as defined in claim 5, wherein said first and second photosensitive means and said light source are so dimensioned and arranged that the part of the light emitted from said source and directed towards said first photosensitive means is greater than the part of the light emitted from said source and directed towards said second photosensitive means.

16. An arrangement as defined in claim 5, wherein said first photosensitive means comprises at least one photoelement having an electrical output and an operational amplifier having an input connected to said electrical output of said photoelement, the output signal of said operational amplifier constituting said first electrical signal.

17. An arrangement as defined in claim 5, wherein said stabilizing circuit means comprises an operational amplifier having an input connected to said first photosensitive means for receipt of said first electrical signal, a source of reference voltage including a Zener diode also connected to said input of said operational amplifier, and said operational amplifier having an output connected to said light source and being operative for varying the luminous flux emitted by said light source in dependence upon the difference in magnitude between the signals at the input of said operational amplifier.

18. An arrangement as defined in claim 12, wherein said first and second photosensitive means and said light source are so dimensioned and arranged that the part of the light emitted from said source and directed towards said first photosensitive means is greater than the part of the light emitted from said source and directed towards said second photosensitive means, and wherein the luminous flux incident upon said second film strip portion decreases in direction towards said edge thereof in such a manner that it is possible that said light-blocking surface portions of said second film strip portion are sufficiently wide for small changes of said second electrical signal.

19. An arrangement as defined in claim 18, wherein said first photosensitive means comprises a first photoelement and a first operational amplifier having an input connected to the output of said first photoelement, the output signal of said first operational amplifier constituting said first electrical signal, and said operational amplifier including a first resistor connected between the output and input thereof, and wherein said stabilizing circuit means comprises a second operational amplifier having an input connected to the output of said first operational amplifier for receipt of said first electrical signal, a source of reference voltage including a Zener diode and a second resistor connecting said Zener diode to said input of said second operational amplifier, said second operational amplifier having an output connected to said light source and being operative for varying the luminous flux emitted by said light source in dependence upon the difference in magnitude between the signals at said input of said second operational amplifier, and wherein the distribution of light from said source onto said first and second photoelements corresponds to the ratio of the maximum output voltage of said first operational amplifier to the voltage of said Zener diode in such a manner that said first resistor is equal in value to said second resistor to effect a temperature compensation. 

1. An arrangement of the character described, comprising, in combination, a light source; photosensitive means positioned to receive light from said light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from said light source; light-blocking means positioned intermediate said light source and said photosensitive means and mounted for displacement relative to said photosensitive means and said source in a predetermined direction along a predetermined path, the light transmittance of said light-blocking means varying in said predetermined direction, whereby as said light-blocking means is displaced in said direction the luminous flux received by said photosensitive means from said light source will vary accordingly; and compensating means operative for establishing a predetermined mathematical relationship between the displacement of said light-blocking means and the magnitude of said signal by automatically varying the luminous flux emitted from said light source towards said photosensitive means in dependence upon the position of said light-blocking means, wherein said photosensitive means and said signal respectively constitute first photosensitive means and a first signal, and wherein said compensating means comprises second photosensitive means positioned to receive light from said light source and operative for generating a second electrical signal having a magnitude dependent upon the luminous flux received from said light source, and stabilizing circuit means connected to said second photosensitive means and to said light source, and operative for maintaining constant the magnitude of said second signal by automatically varying the luminous flux emitted by said light source, said light-blocking means constituting first light-blocking means, and wherein said compensating means further includes second light-blocking means positioned intermediate said light source and said second photosensitive means and mounted for displacement relative to said second photosensitive means and said source in a predetermined direction along a predetermined path, said second light-blocking means being coupled to said first light-blocking means for displacement jointly therewith, and the light transmittance of said second light-blocking means varying in the direction in which said second light-blocking means is displaceable.
 2. An arrangemenT of the character described, comprising, in combination, a light source; photosensitive means positioned to receive light from said light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from said light source; light-blocking means positioned intermediate said light source and said photosensitive means and mounted for displacement relative to said photosensitive means and said source in a predetermined direction along a predetermined path, the light transmittance of said light-blocking means varying in said predetermined direction, whereby as said light-blocking means is displaced in said direction the luminous flux received by said photosensitive means from said light source will vary accordingly; and compensating means operative for establishing a linear relationship between the displacement of said light-blocking means and the magnitude of said signal by automatically changing both the luminous flux emitted from said light source and the luminous flux incident upon said light-blocking means in dependence upon the position of said light-blocking means.
 3. An arrangement of the character described, comprising, in combination, a light source; photosensitive means positioned to receive light from said light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from said light source; light-blocking means positioned intermediate said light source and said photosensitive means and mounted for displacement relative to said photosensitive means and said source in a predetermined direction along a predetermined path, the light transmittance of said light-blocking means varying in said predetermined direction, whereby as said light-blocking means is displaced in said direction the luminous flux received by said photosensitive means from said light source will vary accordingly; and compensating means operative for establishing a predetermined mathematical relationship between the displacement of said light-blocking means and the magnitude of said signal by automatically changing both the luminous flux emitted from said light source and the luminous flux incident upon said light-blocking means in dependence upon the position of said light-blocking means.
 4. An arrangement as defined in claim 3, wherein said photosensitive means and said signal respectively constitute first photosensitive means and a first signal, and wherein said compensating means comprises second photosensitive means positioned to receive light from said light source and operative for generating a second electrical signal having a magnitude dependent upon the luminous flux received from said light source, and stabilizing circuit means connected to said second photosensitive means and to said light source, and operative for maintaining constant the magnitude of said second signal by automatically varying the luminous flux emitted by said light source.
 5. An arrangement of the character described, comprising, in combination, a light source; photosensitive means positioned to receive light from said light source and operative for generating an electrical output signal having a magnitude dependent upon the luminous flux received from said light source; light-blocking means positioned intermediate said light source and said photosensitive means and mounted for displacement relative to said photosensitive means and said source in a predetermined direction along a predetermined path, the light transmittance of said light-blocking means varying in said predetermined direction, whereby as said light-blocking means is displaced in said direction the luminous flux received by said photosensitive means from said light source will vary accordingly; and compensating means operative for establishing a linear relationship between the displacement of said light-blocking means and the magnitude of said signal by automatically varying the luminous flux emitted from said liGht source towards said photosensitive means in dependence upon the position of said light-blocking means, wherein said photosensitive means and said signal respectively constitute first photosensitive means and a first signal, and wherein said compensating means comprises second photosensitive means positioned to receive light from said light source and operative for generating a second electrical signal having a magnitude dependent upon the luminous flux received from said light source, and stabilizing circuit means connected to said second photosensitive means and to said light source, and operative for maintaining constant the magnitude of said second signal by automatically varying the luminous flux emitted by said light source, said light-blocking means constituting first light-blocking means, and wherein said compensating means further includes second light-blocking means positioned intermediate said light source and said second photosensitive means and mounted for displacement relative to said second photosensitive means and said source in a predetermined direction along a predetermined path, said second light-blocking means being coupled to said first lightblocking means for displacement jointly therewith, and the light transmittance of said second light-blocking means varying in the direction in which said second light-blocking means is displaceable.
 6. An arrangement as defined in claim 5, wherein the relationship between the magnitude of said first and second signals and the luminous flux received by said first and second photosensitive means, respectively, is temperature-dependent, and further including means for maintaining said first and second photosensitive means at substantially the same temperature.
 7. An arrangement as defined in claim 5, wherein the variation of the light transmittance of said first photosensitive means in the direction of displaceability thereof is linear.
 8. An arrangement as defined in claim 5, wherein said first and second blocking means are comprised of a film strip.
 9. An arrangement as defined in claim 5, wherein said first blocking means is comprised of a first film strip and wherein said second blocking means is comprised of a second film strip, and wherein said film strips are mounted on a common support structure for movement in the same direction.
 10. An arrangement as defined in claim 5, wherein said first and second blocking means are respectively comprised of first and second coiled film strip portions supported at the edges thereof on two axially spaced disks mounted on a rotating shaft, said coiled film strip portions together with said disks defining a chamber, and wherein said light source is located interiorly of said chamber and wherein said first and second photosensitive means are located exteriorly of said chamber.
 11. An arrangement as defined in claim 10, wherein said light source comprises a gallium arsenide light-emitting diode.
 12. An arrangement as defined in claim 9, wherein said first film strip portion has a plurality of identical wedge-shaped light-blocking surface portions, and wherein said second film strip portion has along one edge thereof an elongated light-blocking surface portion of varying width.
 13. An arrangement as defined in claim 5, wherein said first and second photosensitive means comprise photoelements mounted on a common metal support, and including means intermediate said first and second photosensitive means for shielding the latter from stray light.
 14. An arrangement as defined in claim 5, wherein said first and second photosensitive means each comprise silicon semiconductor photoelements.
 15. An arrangement as defined in claim 5, wherein said first and second photosensitive means and said light source are so dimensioned and arranged that the part of the light emitted from said source and directed towards said first photosensitive means is greater than the part of the light emitted from said source and directed towards said second phOtosensitive means.
 16. An arrangement as defined in claim 5, wherein said first photosensitive means comprises at least one photoelement having an electrical output and an operational amplifier having an input connected to said electrical output of said photoelement, the output signal of said operational amplifier constituting said first electrical signal.
 17. An arrangement as defined in claim 5, wherein said stabilizing circuit means comprises an operational amplifier having an input connected to said first photosensitive means for receipt of said first electrical signal, a source of reference voltage including a Zener diode also connected to said input of said operational amplifier, and said operational amplifier having an output connected to said light source and being operative for varying the luminous flux emitted by said light source in dependence upon the difference in magnitude between the signals at the input of said operational amplifier.
 18. An arrangement as defined in claim 12, wherein said first and second photosensitive means and said light source are so dimensioned and arranged that the part of the light emitted from said source and directed towards said first photosensitive means is greater than the part of the light emitted from said source and directed towards said second photosensitive means, and wherein the luminous flux incident upon said second film strip portion decreases in direction towards said edge thereof in such a manner that it is possible that said light-blocking surface portions of said second film strip portion are sufficiently wide for small changes of said second electrical signal.
 19. An arrangement as defined in claim 18, wherein said first photosensitive means comprises a first photoelement and a first operational amplifier having an input connected to the output of said first photoelement, the output signal of said first operational amplifier constituting said first electrical signal, and said operational amplifier including a first resistor connected between the output and input thereof, and wherein said stabilizing circuit means comprises a second operational amplifier having an input connected to the output of said first operational amplifier for receipt of said first electrical signal, a source of reference voltage including a Zener diode and a second resistor connecting said Zener diode to said input of said second operational amplifier, said second operational amplifier having an output connected to said light source and being operative for varying the luminous flux emitted by said light source in dependence upon the difference in magnitude between the signals at said input of said second operational amplifier, and wherein the distribution of light from said source onto said first and second photoelements corresponds to the ratio of the maximum output voltage of said first operational amplifier to the voltage of said Zener diode in such a manner that said first resistor is equal in value to said second resistor to effect a temperature compensation. 